A brushless motor having the construction disclosed e.g. in Japanese Utility Model Laying-Open Publication No. 59-161383 is known as a prior art brushless motor which is shown in FIG. 7 of this application.
In this motor, the center hole 43a of a core 43 is fitted to a step portion 42d of a rotor holder 42 and the core and the rotor holder 42 are secured to each other by means of screws 44.
However, according to this prior art technique, since the core 43 is directly in contact with the rotor holder 42 and a frame 41, it gave rise to a problem that vibration noise due to switching of a coil, etc. are transmitted from the core 43 to the exterior through the rotor holder 42 which produces extraordinary vibration.
In order to remove this problem, the applicants of this application have studied motors having constructions indicated in FIGS. 5 and 6.
In FIG. 5, reference numeral 8 indicates a flange-forming frame having a sleeve 9 (bearing holder) at its central portion. A shaft 21 is supported rotatably by this sleeve 9 on the inner side thereof through upper and lower bearings 22 and 23. A supporter 10 is secured to the shaft 21 at its upper end by inserting it with pressure in the shaft hole of the supporter. An upside down laid cup-shaped rotor case 32 is secured to this supporter 10. A ring-shaped rotor magnet 33 is secured to this rotor case 32 on the inner surface thereof. On the frame 8 stated above a rubber plate 31 serving as a vibration isolator is laid down by using a sleeve 9 as a guide. An armature core 1 consisting of a plurality of magnetic plates superposed on each other is laid down similarly by using the sleeve 9 as a guide. A driving coil 30 is wound around the armature core 1. Screws 27 traverse the armature core 1 and the rubber plate 31 described above. The armature core 1 is fixed between the head portions 28 of the screws 27 and the frame 8 through the rubber plate 31 by screwing the screw into the frame 8. The rubber plate 31 has also a noise reducing function. The outer peripheral surface of the armature core 1 is opposite to the inner surface of the rotor magnet 33 with a predetermined interval, comprising in this way an outer rotor type brushless motor. A bottom plate 26 is secured to the bottom surface of the frame 8 and disposed so as to be opposite to the free end surface of the rotor case 32.
According to the structure described above, drive noise and vibrations are prevented from being transmitted to the exterior, because the rubber plate 31 absorbs the drive noise stated above such as switching noise of the coil 30, control noise, etc. as well as the vibrations.
FIG. 6 shows another example of the brushless motors which the applicants of this application have studied. This example differs from that indicated in FIG. 5 only in that apart from the rubber plate 31 stated above serving as a vibration isolator, another rubber plate 35 serving as a vibration isolator is disposed also between the upper surface of the armature core 1 and the head portions of the screws 27 and the armature core 1 is fixed on the frame 8 through this rubber plate 35 by means of the screws 27, and the other construction is identical to that of the example indicated in FIG. 5.
In the case indicated in FIG. 6 also, the rubber plate 31 and the other rubber plate 35 prevent drive noise such as switching noise, control noise, etc. as well as vibrations from being transmitted to the exterior.
In the brushless motors indicated in FIGS. 5 and 6, as described above, the compressive force of the rubber plates 31, 35 serving as vibration isolators varies, depending on the clamping torque of the screws 27 and therefore they have a problematical point that the noise preventing and vibration isolating effect fluctuates, depending on the clamping torque of the screws 27. There is another problem in that if the compressive force of the vibration isolators is reduced in order to increase the vibration isolating effect, the clamping torque by means of the clamping members is apt to be insufficient and if the clamping torque by means of the clamping members is insufficient, the screws 27 are loosened.